1. Technical Field
The present invention relates to a method of manufacturing a semiconductor device.
2. Related Art
In recent years, a size of a semiconductor device has decreased. In the semiconductor devices, an element isolating film having an STI (Shallow Trench Isolation) structure has been used as a structure that isolates individual elements from each other. The STI is formed by providing a trench (hereinafter, referred to as an “STI trench”) using a silicon nitride film pattern as a mask and embedding an insulating film, such as a silicon oxide film, in the STI trench, in a semiconductor substrate.
The insulating film is formed using a chemical vapor deposition (CVD) method. However, before depositing the insulating film using the CVD method, a bottom surface of the STI trench is thermally oxidized and a thin silicon oxide film is formed on the surface of the STI trench.
Since a size of a semiconductor device decreases, a ratio (aspect ratio) of a depth of the STI trench to an opening width increases. If the aspect ratio increases, an embedding property of the insulating film is deteriorated. As a result, an upper end of the STI trench may be embedded by the insulating film before the entire STI trench is embedded by the insulating film, which results in forming a void in the STI trench.
An example of a method that improves deterioration of a embedding property due to a decrease in the size is disclosed in U.S. Pat. No. 6,271,143. According to a technique that is described in U.S. Pat. No. 6,271,143, a method that expands (pull-back) an opening of an insulating film used as a mask at the time of forming the STI trench is used, and an upper end of the opening is suppressed from being clogged by increasing a width of the opening by etching In this way, a void can be suppressed from being formed in the STI trench.
Further, Japanese Laid-open patent publication NO. 8-288382 discloses a method in which an organic film that is composed of an organic polymer is formed on side surfaces and a bottom surface of the STI trench, and the organic film of the bottom surface is removed. According to this method, anisotropy can be maintained at a film deposition speed of the insulating film.
Furthermore, according to a technique that is disclosed in Japanese Laid-open patent publication NO. 2004-207280, an embedding property of an STI trench is not improved but adhesion of an insulating film that embeds the STI trench is improved. Accordingly, contaminated materials or residual materials that are attached to a surface where the insulating film is formed can be removed by a sputtering effect of ions that are present in plasma.
However, if the techniques that are disclosed in U.S. Pat. No. 6,271,143, Japanese Laid-open patent publication NO. 8-288382, and Japanese Laid-open patent publication NO. 2004-207280 are applied to the method of manufacturing a semiconductor device having a small size, the following problems are generated.
First, according to the technique that is disclosed in U.S. Pat. No. 6,271,143, if a size of a semiconductor element decreases, intervals between the STI trenches are also narrowed. For this reason, an opening width of the STI trench cannot be made to be equal to a width, which is needed to effectively embed the insulating film until the bottom surface of the STI trench. Further, if the opening width of the STI trench excessively increases, a substrate is exposed, and a threshold voltage adjusting ion implantation depth may be different. As a result, transistors that effectively have two kinds of threshold voltages are present in parallel, which results in deteriorating an element characteristic.
Further, according to the technique that is disclosed in Japanese Laid-open patent publication NO. 8-288382, if the size of the semiconductor device decreases, an opening width of the STI trench is also narrowed. As a result, it may be difficult for etching species to reach the bottom portion of the STI trench. For this reason, a removal speed of an organic film that is formed on side surfaces of the STI trench may be faster than a removal speed of an organic film that is formed on a bottom surface of the STI trench, which makes it impossible to remove only the organic film on the bottom surface by etching. In this case, anisotropy cannot be maintained at a film deposition speed of the insulating film, and the insulting film cannot be uniformly embedded on the side surfaces and the bottom surface of the STI trench.
As described above, according to the techniques that are disclosed in U.S. Pat. No. 6,271,143, Japanese Laid-open patent publication NO. 8-288382, and Japanese Laid-open patent publication NO. 2004-207280, it is not possible to embed the insulating film without forming the void in the minute STI trench.